This invention provides a new class of highly reactive vinyl-polymerizable monomers which have properties characteristic of surfactants.
It is well known that surfactants, having both hydrophilic and lipophilic moieties, can be made by balancing these dual functionalities, to serve as emulsifiers to make emulsions of oils in water.
In particular, some surfactants can function as effective agents in the free-radical catalyzed conversion of aqueous emulsions of certain olefinically unsaturated monomers into stable dispersions or latexes of polymer particles. In such processes of emulsion polymerization, present surfactants can, in varying degrees of satisfaction, perform several functions. They can maintain a reservoir of unreacted monomer in the form of emulsified droplets; they can generate sites of polymerization by forming micelles in the aqueous medium; and they can give colloidal stability to the polymerized latex particles by orienting their molecules at the polymer-water interface.
Latexes produced by emulsion polymerization have many uses, particularly as coatings either by themselves or in formulation with pigments and other additives. Thus, they are used, for example, in paints and floor-polishes, and in coatings applied to paper, paperboard, plastic films and the like.
However, when present conventional surfactants are used in effecting emulsion polymerization, inherent difficulties are encountered which are well-known in the art. The very steps which are taken to prepare a stable latex can also lead to deficiencies in performance of the latex in subsequent application. Thus, for example, it is highly desirable in many industrial applications to have latexes with very fine particle size and latexes are therefore commonly prepared by using an amount of surfactant sufficiently large to generate many sites of polymerization. It is then usually necessary to add still more surfactant later to give colloidal stability to the growing particles.
But when the resultant latex is coated and dried on a substrate such as paper, the excessive surfactant in the drying latex does not remain distributed uniformly between the coalescing polymer particles. Instead, the molecules of surfactant exude to the surface of the coating, giving rise to undesirable conditions variously characterized as blooming, blushing and the like. Even when the migrating surfactant molecules do not immediately give an unsightly appearance to the coating, subsequent exposure to moisture may result in "water-spotting". Such behavior is particularly undesirable when the latex is a component of a floor polish or paint.
Various attempts have been made to obtain stable latexes without requiring excess amounts of surfactant. Thus, polar comonomers have been used which, after copolymerization, can contribute to the hydrophilic-lipophilic balance on the latex particles. However, when vinyl sulfonate or various carboxylate monomers such as acrylic or methacrylic acid are used for such purpose, conventional surfactants are generally still required to generate micelles in which the emulsion polymerization can be initiated; and it is extremely difficult to adjust the proportion of such polar comonomers to obtain the desired end-properties of copolymer without having to resort to post-added conventional surfactant for colloidal stability.